Co-operative modes of motion in liquid lead

Co-operative modes of motion in liquid lead have been observed using the acceleration of slow neutrons. The frequency-wave number relationship for the modes has been derived and is similar to that found in solid lead. The lifetime or interaction of the modes has been studied by fitting a number of models to the data. It is concluded that for wave numbers less than 2 ?-1 the modes can be described by an extension of hydrodynamic theory to high frequencies, by admitting a sufficiently complicated damping function. A damping function which involves both solid-like and fluid-like terms is suggested.